1. Field of the Invention
The present invention relates to a voltage controlled oscillator, and a PLL circuit and a wireless communication apparatus which employ the same. More particularly, the present invention relates to a voltage controlled oscillator having a band switching function, and a PLL circuit and a wireless communication apparatus which employ the same.
2. Description of the Background Art
Voltage control oscillators are widely used as means for generating a local oscillation signal for wireless communication apparatuses. The voltage controlled oscillator, when manufactured as a high-frequency IC, requires a wide oscillation frequency range to tolerate variations in the components caused by the semiconductor manufacturing process. Recently, there is a demand for a voltage controlled oscillator which has an oscillation frequency variable over a wide frequency range in order to support a communication system which employs different frequency bands.
FIG. 13 is a diagram illustrating an exemplary structure of a conventional voltage controlled oscillator 500 having a band switching function. In FIG. 13, the conventional voltage controlled oscillator 500 includes inductors 501, 502, a power source terminal 503, variable capacitance elements 504, 505, a control voltage terminal 506, oscillation transistors 507, 508, a current source 509, capacitive elements 511, 512, switching elements 513, 514, and a control voltage terminal 515. In FIG. 13, a bias circuit and the like are not illustrated.
Hereinafter, an operation of the conventional voltage controlled oscillator will be described with reference to FIG. 13. In the voltage controlled oscillator 500 of FIG. 13, the inductors 501, 502 are connected in series, and the power source terminal 503 for supplying a power source Vdd is connected between the inductor 501 and the inductor 502. The inductors 501, 502 and the power source terminal 503 constitute an inductor circuit 520. The control voltage terminal 506 is connected to a connection point of the variable capacitance element 504 and the variable capacitance element 505. The variable capacitance elements 504, 505 and the control voltage terminal 506 constitute a variable capacitance circuit 530. The two oscillation transistors 507, 508 are cross-coupled, constituting a negative resistance circuit 540. The capacitive elements 511, 512 and the switching elements 513, 514 constitute a high-frequency switch circuit (band switching circuit) 510. The control voltage terminal 515 is connected to a connection point of the switching element 513 and the switching element 514 to supply a control voltage to the switching elements 513, 514.
The sources of the oscillation transistors 507, 508 are connected to each other and are also connected to one terminal of the current source 509. The other terminal of the current source 509 is grounded. One terminal of each of the switching elements 513, 514 is connected to the capacitive element 511, 512, respectively, another terminal thereof is grounded, and still another terminal thereof is connected to the control voltage terminal 515.
The power source Vdd is supplied from the power source terminal 503 via the inductors 501, 502 to the oscillation transistors 507, 508, respectively. An output of one of the oscillation transistors 507, 508 is fed back to the gate of the other transistor. Thereby, the oscillation circuit oscillates in the vicinity of a resonance frequency which is determined by a parallel resonance circuit composed of the inductor circuit 520, the variable capacitance circuit 530, and the high-frequency switch circuit 510. Specifically, a differential voltage between a control voltage Vt input from the control voltage terminal 506 and the power source Vdd is applied across each of the variable capacitance elements 504, 505. The variable capacitance elements 504, 505 have a capacitance which is determined, depending on the differential voltage. Therefore, the oscillation frequency varies, depending on the control voltage Vt input from the control voltage terminal 506. The switching elements 513, 514 are switched ON/OFF, depending on a control voltage Vctrl input from the control voltage terminal 515, so that a capacitance value of the whole band switching circuit 510 is determined. Therefore, by switching ON/OFF the switching elements 513, 514, the oscillation frequency can be shifted.
FIG. 14A is a diagram illustrating how the oscillation frequency is shifted in the conventional voltage controlled oscillator. Typically, the conventional voltage controlled oscillator employs a plurality of high-frequency switch circuits in order to obtain a wide range in which the oscillation frequency varies. In FIG. 14A, there are nine bands. Thus, in the voltage controlled oscillator 500 of FIG. 13, the oscillation frequency can be continuously changed by controlling the control voltage Vt, and in addition, the oscillation frequency band can be changed by controlling the control voltage Vctrl.
The voltage controlled oscillator 500 of FIG. 13 has an oscillation frequency of f0 represented by:f0=1/(2π(2L·C′/2)1/2)=1/(2π(L·C′)1/2), andC′=C1+C2+C3where L represents an inductance of each of the inductors 501, 502, C1 represents a capacitance value of each of the variable capacitance elements 504, 505, C2 represents a capacitance value of each of the capacitive elements 511, 512 of the high-frequency switch circuit, and C3 represents the remaining differential parasitic capacitance component.
When switched OFF, the switching elements 513, 514 are interrupted, and therefore, the capacitive elements 511, 512 are not connected to the resonant circuit with respect to a high frequency signal. Therefore, in this case, the oscillation frequency f0_off is represented by:f0_off=1/(2π(L·(C1+C3))1/2).
On the other hand, when switched ON, the switching elements 513, 514 are brought into conduction, and therefore, the capacitive elements 511, 512 are connected to the resonant circuit with respect to a high-frequency signal. Therefore, in this case, the oscillation frequency f0_on is represented by:f0_on=1/(2π(L·(C1+C2+C3))1/2).
As used herein, the term “frequency tuning sensitivity” refers to a ratio of a change in the oscillation frequency to the control voltage Vt. The frequency tuning sensitivity is determined based on a ratio of a change amount in capacitance of the variable capacitance circuit to the total capacitance value of the resonant circuit. The frequency tuning sensitivity is increased with an increase in this ratio. f0_off has a higher frequency tuning sensitivity than that of f0_on.
Thus, a higher oscillation frequency and a higher frequency tuning sensitivity are obtained when the switching elements 513, 514 are switched OFF.
As illustrated in FIG. 14A, as the number of bands is increased by using an increased number of switching elements in the high-frequency switch circuit, a difference in frequency tuning sensitivity between the highest oscillation frequency band and the lowest oscillation frequency band increases.
However, a relationship between the control voltage Vt and the oscillation frequency of the voltage controlled oscillator is preferable when all of the bands, i.e., all of the oscillation frequencies have substantially the same slope. This is because, when the voltage controlled oscillator is used to construct a phase lock loop (PLL) circuit, transient response characteristics or noise-band characteristics of of the PLL circuit depends on the frequency tuning sensitivity with respect to the control voltage, and therefore, when the frequency tuning sensitivity varies depending on the frequency, characteristics of the PLL circuit itself varies depending on the frequency.
When the voltage controlled oscillator is implemented on a semiconductor substrate, the oscillation frequency also needs to be variable over a wide frequency range. In the conventional voltage controlled oscillator 500 of FIG. 13, a high-frequency switch circuit can be used to obtain a wide range of variable frequency. However, it is difficult to obtain substantially the same frequency tuning sensitivity over the entire wide variable-frequency range.
To solve the above-described problems, some circuits have already been proposed (see, for example, Japanese Patent Laid-Open Publication Nos. 2003-174320 and 2004-15387).
FIG. 15 is a circuit diagram illustrating a conventional voltage controlled oscillator 600 which employs an improved method of obtaining substantially the same frequency tuning sensitivity over a wide range of variable frequency.
In FIG. 15, parts having a function similar to that of the conventional voltage controlled oscillator 500 of FIG. 13 are referenced with the same reference numerals and will not be explained. In FIG. 15, the conventional voltage controlled oscillator 600 is different from the conventional voltage controlled oscillator 500 in that the conventional voltage controlled oscillator 600 includes variable capacitance elements 551, 552, 561, 562, 571, 572 and switching elements 553, 554, 563, 564, 573, 574.
In the voltage controlled oscillator 600 of FIG. 15, first, second and third variable capacitance circuits 550, 560, 570 are connected in parallel. In the first variable capacitance circuit 550, switching elements 553, 554 are connected to opposite ends of series-connected variable capacitance elements 551, 552. In the second variable capacitance circuit 560, switching elements 563, 564 are connected to opposite ends of series-connected variable capacitance elements 561, 562. In the third variable capacitance circuit 570, switching elements 573, 574 are connected to opposite ends of series-connected variable capacitance elements 571, 572. The first, second and third variable capacitance circuits 550, 560, 570 receive a control voltage Vt through a control voltage terminal 506.
As the oscillation frequency is lowered, a ratio of the capacitance change amount of the variable capacitance circuit to the total capacitance value of the resonant circuit is decreased, resulting in a decrease in the frequency tuning sensitivity. Therefore, for a band having a lower oscillation frequency, by increasing the number of variable capacitance circuits connected to the resonant circuit by switching ON the switching elements in the variable capacitance circuits to increase the change amounts in capacitance of the variable capacitance elements as described below, the frequency tuning sensitivity can be caused to be substantially the same as when the oscillation frequency is high.
For example, to provide a band having the highest oscillation frequency of FIG. 14A, the switching elements 553, 554 of only the first variable capacitance circuit 550 among the first to third variable capacitance circuits 550, 560, 570 are switched ON while the others are switched OFF. Also, for example, to provide a band having the fifth oscillation frequency counted from the top in FIG. 14A, the switching elements 553, 554, 563, 564 of the first and second variable capacitance circuits 550, 560 among the first to third variable capacitance circuits 550, 560, 570 are switched ON while the others are switched OFF. Also, for example, to provide a band having the lowest oscillation frequency of FIG. 14A, the switching elements 553, 554, 563, 564, 573, 574 of all of the first to third variable capacitance circuits 550, 560, 570 are switched ON. Thus, as the oscillation frequency is decreased, the number of variable capacitance circuits connected in parallel in the resonant circuit is increased, thereby increasing a change amount in the capacitance. As a result, it is possible to obtain substantially the same frequency tuning sensitivity over the entire oscillation frequency range. FIG. 14B is a diagram illustrating characteristics of the voltage controlled oscillator 600 when substantially the same frequency tuning sensitivity is obtained over the entire oscillation frequency range.
However, in the conventional improved method of FIG. 15, the switching elements 553, 554, 563, 564, 573, 574 are provided in portions of the variable capacitance circuits 550, 560, 570 through which a high-frequency signal flows, and therefore, phase noise characteristics are deteriorated due to loss in the switching elements. For example, when the switching element is a MOS switch, loss occurs in the on-resistance, resulting in a deterioration in the phase noise characteristics.
Japanese Patent Laid-Open Publication No. 2003-324316 discloses a voltage controlled oscillator for use in prevention of deterioration of the phase noise characteristics. FIG. 16A is a circuit diagram illustrating a conventional voltage controlled oscillator 700 described in the document. FIG. 16B is a graph illustrating characteristics of the conventional voltage controlled oscillator 700 of the document. In the conventional voltage controlled oscillator 700, variable capacitance elements and an inductor 707 are connected in parallel. The conventional voltage controlled oscillator 700 switches ON/OFF switching elements 708 to 711, thereby changing the number of variable capacitance elements. As a result, the capacitance of an LC oscillator is changed, thereby changing oscillation frequency bands. In FIG. 16B, A indicates characteristics when a control voltage CTRL is applied only to a middle point between the variable capacitance elements 701 and 702 while Vss is applied to a middle point between the variable capacitance elements 703 and 704 and a middle point between the variable capacitance elements 705 and 706; B indicates characteristics when only one of the switching elements 708 and 710 is switched ON; and C indicates characteristics when both the switching elements 708 and 710 are switched ON. For the band A, although the total capacitance of the resonant circuit is largest (the lowest oscillation frequency), the ratio of the capacitance change amount of the variable capacitance circuit to the total capacitance is small. Therefore, the frequency tuning sensitivity is lowest for the band A. Thus, the frequency tuning sensitivity varies from band to band.